Fast rhythm of life and everyday stress has raised significantly the role of mental diseases and disorders in our society. Depression and other mental disorders are more and more frequent. About 340 million people (6 percent of total population of the world) suffer from deep depression. According to study performed by NIH (National Institute of Health), USA, during last 10 years the number of diagnosed depression increased about 40 times.
Even though depression and other mental disorders are more frequent, the physiological mechanisms of these are not finally clear yet. As a cause of depression, biochemical changes in brain can be considered as disturbance of the function of catecholamines and serotonin in the brain. According to another theory, depression is related to the imbalance of neurotransmitters in brain.
The diagnosis for depression is based on evaluation of the intensity of subjective and clinical symptoms by psychiatrists (M.I.N.I. interview, Hamilton test, and others). Distinguishing reactions to somatic diseases from depressive disorders requiring treatment is very complicated in psychiatric diagnostics. Therefore, there is a great need for methods for determining depression based on objective symptoms. There is a further need for objective monitoring of possible appearance of depressive conditions or other mental disorders of high-risk or high-stress workers such as military personnel, police, rescue workers.
Without doubt, changes in physiological state of the brain do occur together with mental illness and analysis of such changes can provide objective information. Changes in rhythms of bioelectrical activity of the brain, related to the changes in EEG, have been successfully used for diagnosing several neurological and psychiatric diseases (epilepsy, schizophrenia and others). However, only limited data are available about changes in neurophysiological state of the brain in depressive disorders.
Based on previous studies it was supposed that left frontal hypoactivation is distinctive for depressed individuals, being characterized by relatively more left alpha activity [1-4]. Moreover, frontal alpha asymmetry seems to characterize also recovered depressives [1, 3]. The results of the studies showed that EEG alpha asymmetry in depressives demonstrates stability that is comparable in magnitude to that seen in non-clinical populations and the stability is apparent despite rather substantial improvements in clinical state [2].
No relationship between depression severity and EEG asymmetry could be proven [3]. In addition, absolute and relative power in beta band appeared to differentiate patients and controls, with patients exhibiting more power than controls [1]. One of the studies employing LORETA (low-resolution electromagnetic tomography) observed in depressed individuals a pattern of more central, temporal, superior fronto-lateral and medial frontal asymmetry (increased alpha2 current density in the left hemisphere as compared to the right hemisphere) [4], which correlates with previous findings. Decreased current density in delta band was observed in right temporal lobe and the same trend was seen also in theta, alpha and beta band [4].
The results of the other study, employing the same method, showed increased source-current density underlying the EEG from the right hemisphere in the delta, alpha and beta frequency bands both during the resting and cognitively challenged conditions [3]. The expected left anterior hypoactivation in depression (reflected by increased resting left frontal EEG power in the alpha band compared to controls) was not seen [3]. The results suggested exactly the opposite, increased activation of the left frontal lobe and decreased activation of the right frontal lobe. Results showed significantly reduced delta band source-current density in depressed individuals compared to controls during the resting condition in most of the brain volume [3].
The following references were addressed:    [1] V. Knott, C. Mahoney, S. Kennedy, K. Evans, “EEG power, frequency, asymmetry and coherence in male depression,” Psychiatry Research: Neuroimaging, vol. 106, pp. 123-140, 2001.    [2] J. B. Allen, H. L. Urry, S. K. Hitt, and J. A. Coan, “The stability of resting frontal electroencephalographic asymmetry in depression,” Physiology, vol. 41, pp. 269-275, 2004.    [3] P. Flor-Henry, J. C. Lind, Z. J. Koles, “A source-imaging (low-resolution electromagnetic tomography) study of the EEGs from unmedicated males with depression,” Psychiatry Research: Neuroimaging, vol. 109, pp. 191-207, 2004.    [4] J. F. Lubar, M. Congedo, J. H. Askew, “Low-resolution electromagnetic tomography (LORETA) of cerebral activity in chronic depressive disorder,” International Journal of Psychopathology, vol. 49, pp. 175-185, 2003.
From the review above we can conclude that published results about changes in EEG caused by depression are contradictory and do not allow to make useful conclusions to evaluate the depressive mode.
In U.S. Pat. No. 6,021,346 a method for determining positive and negative emotional states by using a relative power in a subband of a specific frequency band increases or decreases in the course of time. The invention determines positive and negative emotional states by using a relative power in a subband of a specific frequency band increases or decreases with the lapse of time. The invention performs a Fourier transform for a unit time not an entire response time regarding a stimulus, and can be used in real time by using a time-frequency analysis method continuously executed with the lapse of time.
In U.S. Pat. No. 6,622,036, neurophysiologic information such as quantitative electroencephalography is used in a method for classifying, diagnosing, and treating physiologic brain imbalances, including for remotely assessing and treating patients with physiologic brain imbalances.
In U.S. Pat. No. 5,230,346, determining the brain condition of a human between normal and abnormal as determined by dementia, and selectively between dementia of the Alzheimer's-type and multi-infarct dementia is effected. Measures of electrical output, spectral ratio and coherence value of the brain are determined. Selected scores are applied to the electrical output, spectral ratio and coherence values. A relationship between the scores and additionally the scored value of a coherence ratio are effected to obtain a diagnostic evaluation.
Depression has been shown to cause asymmetry and decreased coherence between brain hemispheres. However, published results about depression caused changes in EEG asymmetry are contradictory and cause doubts in hemispheric asymmetry based indicators to evaluate the depressive mode. Nevertheless, subjective symptoms of depression should be accompanied by changes in bioelectrical activity of the brain and in the EEG signal. Comparisons of the EEG signals of healthy persons and patients with depressive disorder may allow to detect characteristic features in the EEG produced by depression and to find objective criteria and measures for evaluation of depression and other mental disorders. By the present invention, there is provided a method and device based on the analysis of combination of the EEG selected bands power for distinction of characteristic features in the EEG caused by depression and other mental disorders.